Field of the Invention
The present invention relates to an image capturing apparatus, a control method thereof, and a storage medium.
Description of the Related Art
In current television broadcasting, videos compliant with full high definition (full HD) having the image size of 1920 (H)×1080 (V) are used. Furthermore, broadcasting in SHV (Super High Vision, 8K×4K) having higher definition images than those in full HD size is also scheduled recently. In order to implement an image capturing apparatus compliant with the higher definition images described above, a CMOS sensor having the number of pixels corresponding to 8K (7,680 pixels) in a horizontal direction×4K (4,320 lines) in a vertical direction needs to be employed as an image sensor.
In such a CMOS sensor, an amplifier (column amplifier) and an A/D converter (column A/D converter) are provided for each column. As a consequence, the offset variation of the column amplifier and a vertical line which is one of fixed pattern noise (FPN) components caused by the offset variation of the column A/D converter may occur in a shot image. Moreover, since the above-described CMOS sensor has the number of pixels corresponding to 8K×4K, a phenomenon in which an offset level differs in a horizontal direction may occur as a chip area increases, thereby causing dark (dark time) shading.
In order to solve the above-described problem, it is considered to include, inside the image capturing apparatus, a correction circuit which corrects the vertical line generated for each column.
Signal values output from a VOB (Vertical Optical Black) region serving as a light-shielding pixel region included in the CMOS sensor are averaged in the vertical direction, thereby calculating a VOB value for one vertical column. The calculated average VOB value includes a value for each column. This value is subtracted from each column of an effective pixel, thereby correcting the offset variation for each column.
However, since the above-described correction circuit calculates the VOB value for each column and subtracts the value from an effective pixel for each column, the number of correction circuits increases as the number of horizontal pixels of the CMOS sensor is larger. That is, correction circuits for 7,680 columns are needed to correct the 7,680 pixels in the horizontal direction which achieve SHV. Further, power consumption increases along with an increase in the number of correction circuits.
Therefore, outputting image data without performing vertical line correction processing as described above inside the image capturing apparatus, and then performing correction outside the image capturing apparatus is considered.
When RAW image data is output without performing correction inside the image capturing apparatus, VOB pixel data also needs to be output as a part of the RAW image data. An information amount increases if the VOB image data is added to the RAW image data for all pixels and output. To cope with this, a method of outputting the RAW image data after compression processing is proposed (Japanese Patent Laid-Open No. 11-112885).
In Japanese Patent Laid-Open No. 11-112885, however, a video output is obtained by recording image data in an OB portion and image data in an effective pixel portion after compression processing, and then decompressing them at the time of reproduction. In this case, since the effective pixel portion overlaps the OB portion, the image data in the OB portion has an influence on video near the boundary between the OB portion and the effective pixel portion. To prevent this, an extra pixel is provided between the OB portion and the effective pixel portion. By doing so, the image data in the OB portion has no influence on the video near the boundary between the OB portion and the effective pixel portion even if the image data in the OB portion and the image data in the effective pixel portion are compressed to transmit a signal, and are decompressed at the time of reproduction.
This method is effective to reduce the influence on a main image when compressing the image data in the effective pixel portion and the image data in the OB portion to transmit the signal, and decompressing them at the time of reproduction. However, a problem about vertical line noise specific to the CMOS sensor, a problem which arises when VOB includes a defective pixel, and the like remain unsolved.